This invention relates to improvements in an electron lens for focussing an electron beam emitted from the electron gun used in image forming tubes.
Conventional electron lenses for the electron gun widely used with image forming tubes have been generally divided into the bipotential and unipotential types. At present, the bipotential type of electron guns has been widely adopted particularly in color cathode ray tubes and comprises a cathode, and a first, a second, a third and a fourth grid disposed in the named order on the central axis thereof while the electron lens thereof is disposed on the central axis and is formed of the fourth grid having a high voltage applied thereto and the third grid having applied thereto a moderate voltage equal to about 20% of the high voltage.
In the bipotential type of conventional electron guns as described above, the focussed electron beam spot has been large in diameter in the high current range. For the bipotential type having a triad of electron guns arranged in the delta or the in-line configuration within color image forming tubes, the inside diameter of the neck portion of the tube might limit the aperture of an associated electron lens and increase the spherical aberration thereof because an associated electron gun is disposed in the neck portion. This has resulted in problems because the focussed electron beam spot can not have a sufficiently small spot diameter in the high current range and therefore the resolution is severely decreased. For example, white characters displayed on the phosphor screen of color image forming tubes might become obscure or thick.
On the other hand, the unipotential type of conventional electron guns has been presently employed in one part of color image forming tubes and includes an electron lens formed of a pair of spaced end grids and an intermediate grid interposed therebetween, all the grids being disposed coaxially with one another on the central axis. The end grids have been equal in potential to each other and supplied with equal voltages while the intermediate grid has been supplied with a low voltage kept substantially at the ground potential whereby the resulting potential distribution is saddle-shaped along the central axis of the electron gun.
The unipotential type of conventional electron guns as described above has an increased spherical aberration of the electron lens involved and the focussed beam spot in a high current range includes a small bright core lying at the center thereof and a large dark halo located around the core. This has resulted in a disadvantage because the sharpness becomes bad but the resolution is good while the focussed beam forms a large spot in the low current range and the resolution is deteriorated in that range. In addition, a disadvantage occurs because the dielectric strength characteristics within image forming tubes are adversely affected by such an array of three grids because a high voltage is present on either side of the intermediate grid held at a low potential.
From the foregoing it will be readily understood that both the bipotential type and the unipotential type of conventional electron guns have characteristic features some of which are advantageous and some of which are disadvantageous. Also these conventional electron guns have been unable to increase the resolution over the entire region extending from the low to the high current range and particularly for the latest color image forming tubes which are increasingly operated with high currents at high voltages approximating 30 kilovolts in order to increase the luminescence of the phosphor screen or picture surface thereof.
Accordingly it is an object of the present invention to provide an electron gun including an improved electron lens for increasing the resolution of formed pictures.
It is another object of the present invention to provide an electron gun including an improved electron lens lower in spherical aberration.